﻿/**
* 文本框根据输入内容自适应高度
* @author tang bin
* @version 0.3
* @see http://www.planeart.cn/?p=1489
* @param {HTMLElement} 输入框元素
* @param {Number} 设置光标与输入框保持的距离(默认20)
* @param {Number} 设置最大高度(可选)
*/
var autoTextarea = function (elem, extra, maxHeight) {
    extra = extra || 20;
    var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
    isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
    addEvent = function (type, callback) {
        elem.addEventListener ?
        elem.addEventListener(type, callback, false) :
        elem.attachEvent('on' + type, callback);
    },
    getStyle = elem.currentStyle ? function (name) {
        var val = elem.currentStyle[name];
        if (name === 'height' && val.search(/px/i) !== 1) {
            var rect = elem.getBoundingClientRect();
            return rect.bottom - rect.top - parseFloat(getStyle('paddingTop')) - parseFloat(getStyle('paddingBottom')) + 'px';
        };
        return val;
    } : function (name) {
        return getComputedStyle(elem, null)[name];
    },
    minHeight = parseFloat(getStyle('height'));
    elem.style.maxHeight = elem.style.resize = 'none';
    var change = function () {
        var scrollTop, height,padding = 0,style = elem.style;
        if (elem._length === elem.value.length) return;
        elem._length = elem.value.length;
        if (!isFirefox && !isOpera) {
            padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
        };
        scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
        var historyHeight = parseInt(elem.style.height);
        elem.style.height = minHeight + 'px';
        if (elem.scrollHeight > minHeight) {
            if (maxHeight && elem.scrollHeight > maxHeight) {
                height = maxHeight - padding;
                style.overflowY = 'auto';
            } else {
                height = elem.scrollHeight - padding;
                style.overflowY = 'hidden';
            };
            style.height = height + extra + 'px';
            if (height + extra - historyHeight !=0) {
                historyScrolltop = scrollTop;
                scrollTop = historyScrolltop + height + extra - historyHeight - 14;
                document.body.scrollTop = scrollTop;
                document.documentElement.scrollTop = scrollTop;
            }
            elem.currHeight = parseInt(style.height);
        };
    };
    addEvent('propertychange', change);
    addEvent('input', change);
    addEvent('focus', change);
    change();
};